1. Field of the Invention
The invention generally relates to the early detection of ischemic events such as cardiac ischemia. In particular, the invention provides methods for detecting inosine and/or hypoxanthine, or metabolic by-products thereof, as early biomarkers of ischemia.
2. Background of the Invention
Cardiovascular diseases (e.g. acute myocardial infarction (MI)) are the leading cause of mortality in the world [Naudziunas et al., 2005; Okrainec et al., 2004; Domer et al., 2004, AHRQ, 2000]. Each year in the US, approximately 7-8 million patients present with non-traumatic chest pain and seek emergency medical treatment [Morrow et al., 2007]. Current emergency medical evaluation on these patients suspected of having acute MI includes obtaining patient history, signs and symptoms, vitals, electrocardiogram (ECG) and blood evaluation for specific cardiac biomarkers [Beyerle, 2002; A.D.A.M. Inc., 2005; Lees, 2000]. However, the percent diagnostic accuracy of acute MI when using patient signs and symptoms, ECG and c-troponin is only approximately 50%. With the addition of the recently FDA cleared albumin cobalt binding assay, the diagnostic accuracy improves to approximately 70%; hence the need for additional research for biomarkers of acute cardiac ischemia to further improve patient diagnostic accuracy is important.
The hospital emergency department blood evaluation determines levels of several specific endogenous cardiac protein biomarkers (e.g. cardiac troponin I and T (cTnI, cTnT), creatine kinase-MB (CK-MB) isoform, and myoglobin). However, these protein biomarkers are indicative of cardiac tissue necrosis, and are typically detected hours after the acute cardiac event (infarct), and not at the time of acute cardiac ischemia.
One recent published scientific editorial requested the need for early onset biomarkers of acute cardiac ischemia prior to cardiac tissue necrosis [Morrow et al., 2003]. Ideally, these early onset biomarkers would aid emergency medical services (EMS) personnel in the rapid diagnosis and treatment of initial acute cardiac ischemia (potentially acute MI), thus increasing the survival rate of acute MI victims every year. One research group [Bhagavan et al., 2003] addressing the scientific editorial request, describes a blood measurement for ischemia modified albumin (IMA), which appears at an elevated level in the bloodstream from patients undergoing an ischemic cardiac event; however the author's state that the colorimetric test would not discriminate between cardiac ischemic patients with and without acute MI (e.g. angina), and recent clinical evaluations of the test assay have reported significant false positive results.
This technology is described in U.S. Pat. No. 7,282,369 to Par-Or et al. (Oct. 16, 2007) which teaches rapid methods for the detection of ischemic states and kits for use in such methods. The methods are based on detecting and quantifying the existence of an alteration of the serum protein albumin which occurs following an ischemic event. Methods for detecting and quantifying this alteration include evaluating and quantifying the cobalt binding capacity of circulating albumin, analysis and measurement of the ability of serum albumin to bind exogenous cobalt, detection and measurement of the presence of endogenous copper in a purified albumin sample and use of an immunological assay specific to the altered form of serum albumin which occurs following an ischemic event. Also taught is the detection and measurement of an ischemic event by measuring albumin N-terminal derivatives that arise following an ischemic event, including truncated albumin species lacking one to four N-terminal amino acids or albumin with an acetylated N-terminal Asp residue.
U.S. Pat. No. 7,063,782 to Wayment et al (Jun. 20, 2006) teaches electrochemical methods and devices for in vitro detection of an ischemic event in a patient sample. Following addition of a known amount of a transition metal ion to the patient sample, electrodes are used to measure the current or potential difference of non-sequestered transition metal ion in the sample. The amount of non-sequestered transition metal ion in the sample reflects the degree of modification to albumin that is the result of an ischemic event. However, several clinical studies have reported the test to have significant false positive results.
There is an ongoing need to discover and develop methods for detecting early onset biomarkers of acute cardiac ischemia.